Scattered pilot pattern and channel estimation method for mimo-ofdm systems

ABSTRACT

Methods and apparatus are provided for inserting data symbols and pilot symbols in an OFDM (orthogonal frequency division multiplexing) transmission resource utilizing frequency hopping patterns for the data symbols and/or the pilot symbols. Data symbols and pilot symbols are allocated for down link (base station to mobile station) and up link (mobile station to bases station) transmission resources in a two-dimensional time-frequency pattern. For each antenna of a MIMO-OFDM (multiple input multiple output OFDM) communication system, pilot symbols are inserted in a scattered pattern in time-frequency and data symbols are inserted in an identical frequency-hopping pattern in time-frequency as that of other antennas.

RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 11/529,246 filed Sep. 29, 2006, which is acontinuation-in-part of U.S. patent application Ser. No. 10/038,883filed Jan. 8, 2002 and the present application incorporates the subjectmatter of the two previous applications in their entirety herein byreference. U.S. patent application Ser. No. 10/038,883 claims thebenefit of U.S. Provisional Application No. 60/329,509, filed Oct. 17,2001, the contents of which are also incorporated in their entiretyherein by reference.

FIELD OF THE INVENTION

This invention relates to OFDM communication systems, and moreparticularly to a more efficient use of pilot symbols within suchsystems.

BACKGROUND OF THE INVENTION

Multiple Input Multiple Output—Orthogonal Frequency DivisionMultiplexing (MIMO-OFDM) is a novel highly spectral efficient technologyused to transmit high-speed data through radio channels with fast fadingboth in frequency and in time.

In wireless communication systems that employ OFDM, a transmittertransmits data to a receiver using many sub-carriers in parallel. Thefrequencies of the sub-carriers are orthogonal. Transmitting the data inparallel allows the symbols containing the data to be of longerduration, which reduces the effects of multi-path fading. Theorthogonality of the frequencies allows the sub-carriers to be tightlyspaced, while minimizing inter-carrier interference. At the transmitter,the data is encoded, interleaved, and modulated to form data symbols.Overhead information is added, including pilot symbols, and the symbols(data plus overhead) are organized into OFDM symbols. Each OFDM symboltypically uses 2^(n) frequencies. Each symbol is allocated to representa component of a different orthogonal frequency. An inverse Fast FourierTransform (IFFT) is applied to the OFDM symbol (hence the preference of2^(n) frequencies) to generate time samples of a signal. Cyclicextensions are added to the signal, and the signal is passed through adigital-to-analog converter. Finally, the transmitter transmits thesignal to the receiver along a channel.

When the receiver receives the signal, the inverse operations areperformed. The received signal is passed through an analog-to-digitalconverter, and timing information is then determined. The cyclicextensions are removed from the signal. The receiver performs an FFT onthe received signal to recover the frequency components of the signal,that is, the data symbols. Error correction may be applied to the datasymbols to compensate for variations in phase and amplitude causedduring propagation of the signal along the channel. The data symbols arethen demodulated, de-interleaved, and decoded, to yield the transmitteddata.

In systems employing differential detection, the receiver compares thephase and/or amplitude of each received symbol with an adjacent symbol.The adjacent symbol may be adjacent in the time direction or in thefrequency direction. The receiver recovers the transmitted data bymeasuring the change in phase and/or amplitude between a symbol and theadjacent symbol. If differential detection is used, channel compensationneed not be applied to compensate for variations in phase and amplitudecaused during propagation of the signal. However, in systems employingcoherent detection the receiver must estimate the actual phase andamplitude of the channel response, and channel compensation must beapplied.

The variations in phase and amplitude resulting from propagation alongthe channel are referred to as the channel response. The channelresponse is usually frequency and time dependent. If the receiver candetermine the channel response, the received signal can be corrected tocompensate for the channel degradation. The determination of the channelresponse is called channel estimation. The inclusion of pilot symbols ineach OFDM symbol allows the receiver to carry out channel estimation.The pilot symbols are transmitted with a value known to the receiver.When the receiver receives the OFDM symbol, the receiver compares thereceived value of the pilot symbols with the known transmitted value ofthe pilot symbols to estimate the channel response.

The pilot symbols are overhead, and should be as few in number aspossible in order to maximize the transmission rate of data symbols.Since the channel response can vary with time and with frequency, thepilot symbols are scattered amongst the data symbols to provide ascomplete a range as possible of channel response over time andfrequency. The set of frequencies and times at which pilot symbols areinserted is referred to as a pilot pattern. The optimal temporal spacingbetween the pilot symbols is usually dictated by the maximum anticipatedDoppler frequency, and the optimal frequency spacing between the pilotsymbols is usually dictated by the anticipated delay spread ofmulti-path fading.

The existing pilot-assisted OFDM channel estimation approaches aredesigned for conventional one transmitter system. With a scattered pilotarrangement, there are three classes of algorithms:

-   -   1-D frequency interpolation or time interpolation    -   Transformed frequency 1-D interpolation    -   Independent time and frequency 1-D interpolation

The first class of algorithms is based on the pilot OFDM symbol (all thesub-carriers are used as the pilots) or comb-type of pilots. Thisapproach shown in the flow chart of FIG. 1A is simple but only suitablefor channels with high frequency selectivity or channels with high timefading. The method involves pilot extraction in the frequency domain(step 1A-1) followed by interpolation in time (step 1A-2), orinterpolation in frequency (step 1A-3).

The second method shown in the flow chart of FIG. 1B is aimed forchannels with slow Doppler fading and fast frequency fading. It improvesthe first method by using FFT to reconstruct the channel response backto time domain for noise reduction processing at the expense of FFT/IFFTcomputing for the channel estimation separately. The method begins withpilot extraction in the frequency domain (step 1B-1), which may befollowed by interpolation in frequency (step 1B-2). Then an inverse fastFourier transform (step 1B-3), smoothing/de-noise processing (step1B-4), and finally a fast Fourier transform (1B-5) steps are executed.

The third method shown in the flow chart of FIG. 1C can be used toestimate channel for mobile applications, where both fast time fadingand frequency fading exist. However it needs a relatively high densityof pilots and a completed interpolator. This method involves pilotextraction in the frequency domain (step 1C-1) this is followed byinterpolation in time (step 1C-2) and interpolation in frequency (step1C-3).

In the propagation environment with both high frequency dispersion andtemporal fading, the channel estimation performance can be improved bythe increase of pilot symbol density at the price of the reduction ofthe spectral efficiency of the data transmission. To interpolate andreconstruct the channel response function from the limited pilots toachieve reliable channel estimation with the minimum overhead is achallenging task.

There are a variety of existing standard pilot patterns. In environmentsin which the channel varies only slowly with time and frequency, thepilot symbols may be inserted cyclically, being inserted at an adjacentfrequency after each time interval. In environments in which the channelis highly frequency dependent, the pilot symbols may be insertedperiodically at all frequencies simultaneously. However, such a pilotpattern is only suitable for channels that vary very slowly with time.In environments in which the channel is highly time dependent, the pilotsymbols may be inserted continuously at only specific frequencies in acomb arrangement to provide a constant measurement of the channelresponse. However, such a pilot pattern is only suitable for channelsthat vary slowly with frequency. In environments in which the channel isboth highly frequency and highly time dependent (for example, mobilesystems with much multi-path fading), the pilot symbols may be insertedperiodically in time and in frequency so that the pilot symbols form arectangular lattice when the symbols are depicted in a time-frequencydiagram.

In OFDM communication systems employing coherent modulation anddemodulation, the receiver must estimate the channel response at thefrequencies of all sub-carriers and at all times. Although this requiresmore processing than in systems that employs differential modulation anddemodulation, a significant gain in signal-to-noise ratio can beachieved using coherent modulation and demodulation. The receiverdetermines the channel response at the times and frequencies at whichpilot symbols are inserted into the OFDM symbol, and performsinterpolations to estimate the channel response at the times andfrequencies at which the data symbols are located within the OFDMsymbol. Placing pilot symbols more closely together (in frequency if acomb pattern is used, in time if a periodic pattern is used, or in bothfrequency and in time if a rectangular lattice pattern is used) within apilot pattern results in a more accurate interpolation. However, becausepilot symbols are overhead, a tighter pilot pattern is at the expense ofthe transmitted data rate.

Existing pilot patterns and interpolation techniques are usuallysufficient if the channel varies slowly with time (for example fornomadic applications). However, if the channel varies quickly with time(for example, for mobile applications), the time interval between pilotsymbols must be reduced in order to allow an accurate estimation of thechannel response through interpolation. This increases the overhead inthe signal.

The problem of minimizing the number of pilot symbols while maximizingthe accuracy of the interpolation is also particularly cumbersome inMultiple-Input Multiple-Output (MIMO) OFDM systems. In MIMO OFDMsystems, the transmitter transmits data through more than onetransmitting antenna and the receiver receives data through more thanone receiving antenna. The binary data is usually divided between thetransmitting antennae, although the same data may be transmitted througheach transmitting antenna if spatial diversity is desired. Eachreceiving antenna receives data from all the transmitting antennae, soif there are M transmitting antennae and N receiving antennae, then thesignal will propagate over M×N channels, each of which has its ownchannel response. Each transmitting antenna inserts pilot symbols intothe same sub-carrier location of the OFDM symbol which it istransmitting. In order to minimize interference at the receiver betweenthe pilot symbols of each transmitting antenna, each transmittingantenna typically blinks its pilot pattern on and off. This increasesthe temporal separation of the pilot symbols for each transmitter,reducing the accuracy of the interpolation used to estimate the channelresponse. In MIMO-OFDM systems a simple and fast channel estimationmethod is particularly crucial because of the limitation of thecomputational power for estimating M×N channels, while in SISO-OFDMsystem only one channel needs to be estimated.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a methodfor inserting data and pilot symbols into an Orthogonal FrequencyDivision Multiplexing (OFDM) transmission resource for transmission on Ntransmitting antenna where N≧2, the OFDM transmission resource having atime domain and a frequency domain, each OFDM transmission resourcecomprising a plurality of OFDM symbols, the method comprising the stepsof: for each antenna, inserting pilot symbols in a respective pattern intime-frequency; and inserting data symbols in a frequency-hoppingpattern in time-frequency that is identical for all the antennas,wherein the pilot symbols for each antenna are inserted such that pilotsymbols from other antennas do not occupy the same location intime-frequency.

In some embodiments, inserting data symbols in a frequency-hoppingpattern in time-frequency comprises inserting data symbols on a set ofspaced apart subcarriers that change each symbol duration of a pluralityof symbol durations.

In some embodiments, inserting pilot symbols in a respective pattern intime-frequency comprises: inserting pilot symbols that form at least onediagonal arrangement in time-frequency.

In some embodiments, inserting pilot symbols comprises: when N is equalto two, for each antenna, alternating insertion of null symbol locationsand pilot symbols in the at least one diagonal arrangement for a firstantenna of the pair of antennas and alternating insertion of pilotsymbols and null symbol locations in at least one diagonal arrangementfor a second antenna of the pair of antennas, wherein the null symbollocations of the first antenna correspond to a same location intime-frequency as the pilot symbols of the second antenna, and viceversa.

In some embodiments, inserting pilot symbols comprises: when N is equalto two, for each antenna; inserting pilot symbols in a respective firstdiagonal arrangement in time-frequency, and inserting null symbollocations in a respective second diagonal arrangement in time-frequency,wherein the respective first diagonal arrangement and the respectivesecond diagonal arrangement are parallel and the null symbol locationsof a first antenna of the pair of antennas occur at a same location intime-frequency as the pilot symbols of a second antenna of the pair ofantennas, and vice versa.

In some embodiments, the method further comprises for at least oneantenna of the N transmitting antenna, inserting a larger number ofpilot symbols in time-frequency such that the density of pilot symbolsfor the at least one antenna is higher than for other antennas.

In some embodiments, when the pilot symbols are inserted in a patternhaving at least two diagonal arrangements in time-frequency, the atleast two diagonal arrangements are parallel and offset by a particulardistance in time-frequency.

In some embodiments, the offset between the at least two diagonalarrangements in a direction normal to the diagonal lines is variable fordifferent patterns.

In some embodiments, inserting data symbols on a set of spaced apartsubcarriers comprises: inserting data symbols on a set of spaced apartsubcarriers that are either a set of consecutive spaced apartsubcarriers or a set of subcarriers forming a logical subband ofsubcarriers.

In some embodiments, the OFDM transmission resource is utilized fortransmitting from one or more mobile stations collectively comprisingthe N antennas to a base station.

In some embodiments, inserting data symbols in a frequency-hoppingpattern in time-frequency comprises inserting data symbols on a set ofsubcarriers that is constant over a set of consecutive symbol durations,and change for each set of multiple sets of consecutive symboldurations.

In some embodiments, inserting data symbols on a set of subcarriers thatis constant over a set of consecutive symbol durations comprises:inserting data symbols on a set of subcarriers that are either a set ofconsecutive subcarriers or a set of subcarriers forming a logicalsubband of subcarriers.

In some embodiments, inserting pilot symbols in a respective pattern intime-frequency comprises: for each antenna transmitting a data symbolstream comprising a series of data symbols, by: for each frequency hop,inserting at least one pilot symbol in a corresponding number of OFDMsymbols amongst a plurality of data symbols on a different pair ofsubcarriers of an allocated transmission bandwidth than a pair ofsubcarriers of a previous frequency hop for a previous plurality of datasymbols and at least one pilot symbol of the series of data symbols.

In some embodiments, inserting one or more pilot symbols in a pluralityof OFDM symbols at a different pair of subcarriers comprises: when N isequal to two, for each antenna; inserting a null symbol location andpilot symbol for a first antenna of the pair of antennas and inserting apilot symbol and a null symbol location for a second antenna of the pairof antennas, wherein the null symbol location of the first antenna isinserted at the same location in time-frequency as the pilot symbol ofthe second antenna, and vice versa.

In some embodiments, the method further comprises inserting data andpilot symbols in an OFDM resource for an additional group of Ntransmitting antennas wherein inserting pilot symbols in a respectivepattern in time-frequency for the additional group of N transmittingantennas comprises: employing the same respective pattern of pilotsymbols as the N transmitting antennas where N≧2, but offset in at leastone of time and frequency.

In some embodiments, the method further comprises transmitting the pilotsymbols with a power level greater than a power level of data symbols,depending upon a value reflective of channel conditions.

In some embodiments, adjacent telecommunication cells have differentfrequency-hopping sequences.

In some embodiments, the method further comprises: encoding pilotsymbols using a particular form of pre-processing; transmittinginformation identifying the particular form of pre-processing used toencode the pilot symbols.

According to a second aspect of the invention, there is provided an OFDMtransmitter comprising: a plurality of transmit antennas; an encoderadapted to insert data symbols in an identical frequency-hopping patternin time-frequency for each of the plurality of antennas; a pilotinserter adapted to insert pilot symbols in a respective pattern intime-frequency for each of the antennas, wherein the pilot symbols foreach antenna are inserted such that pilot symbols from other antennas donot occupy the same location in time-frequency.

In some embodiments, the OFDM transmitter is further adapted to: encodepilot symbols using a particular form of pre-processing; transmitinformation identifying the particular form of pre-processing used toencode the pilot symbols.

In some embodiments, the OFDM transmitter is further adapted to transmitthe pilot symbols with a power level that is dynamically adjusted toensure sufficiently accurate reception.

According to a third aspect of the invention, there is provided areceiver comprising: a plurality of transmit antennas for receiving OFDMsymbols including pilot symbols in a respective pattern intime-frequency and data symbols in an identical frequency-hoppingpattern in time-frequency, the pilot symbols for each respective patternin time-frequency inserted such that pilot symbols from differentantennas do not occupy the same location in time-frequency andinformation identifying a particular form of pre-processing used toencode the received pilot symbols from at least one source;differentiating pilot logic adapted to utilize the informationidentifying the particular form of pre-processing used to encode thereceived pilot symbols to differentiate between received pilot symbolsfrom different sources occurring at a same time-frequency location.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe accompanying Figures, in which:

FIG. 1 illustrates flow-charts for three examples of conventional OFDMChannel Estimation;

FIG. 2 is a block diagram of a Multiple-Input Multiple-Output (MIMO)Orthogonal Frequency Division Multiplexing (OFDM) transmitter providedby an embodiment of the invention;

FIG. 3 is a block diagram of an OFDM receiver;

FIG. 4 is a flowchart of a method by which an OFDM transmitter insertspilot symbols into an OFDM frame according to one embodiment of theinvention;

FIG. 5 is a diagram of a pilot pattern generated using the method ofFIG. 4;

FIG. 6 is a block diagram of a MIMO system showing the channel transferfunctions between two transmit antennas and two receive antennas;

FIG. 7 is a time frequency diagram showing channel estimate positionsfor pilot channel estimation;

FIG. 8 schematically illustrates a step of filtering estimated andinterpolated pilot channel estimates;

FIG. 9 shows schematically the step of interpolating between the channelestimates previously determined to provide channel estimates for allsub-carriers and all times;

FIG. 10 is a flow chart summarizing the overall channel estimationmethod provided by an embodiment of the invention;

FIG. 11 is an example of a set of performance results obtained using themethod of FIG. 10;

FIG. 12A is a time-frequency plot showing data mapping for transmissionon downlink antennas using a MIMO OFDM scheme according to an embodimentof the present invention;

FIG. 12B is a time-frequency plot showing data mapping for transmissionon downlink antennas using a MIMO OFDM scheme according to a particulartype of data coding;

FIGS. 13A, 13B, 13C and 13D are time-frequency plots showing pilotsymbol mappings for transmission on downlink antennas using a MIMO OFDMscheme according to embodiments of the invention;

FIGS. 14A and 14B are time-frequency plots showing data symbol and pilotsymbol mapping for transmission on uplink antennas using a MIMO OFDMscheme according to an embodiment of the present invention;

FIG. 15 is a flowchart of a method by which an OFDM transmitter insertsdata symbols and pilot symbols into an OFDM frame according to oneembodiment of the invention;

FIGS. 16A, 16B and 16C are flowcharts of methods used for inserting datasymbols and pilot symbols in time-frequency patterns for DL signalingbetween a base station and one or more mobile stations; and

FIGS. 17A and 17B are flowcharts of methods used for inserting datasymbols and pilot symbols in time-frequency patterns for UL signalingbetween one or more mobile stations and a base station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following sections describe a MIMO-OFDM transmitter/receiver andscattered pilot insertion. By way of introduction, an OFDM frameconsists of the preamble OFDM symbols and regular OFDM symbols. EachOFDM symbol uses a set of orthogonal sub-carriers. When there are twotransmit antennas, two OFDM symbols form a STTD block. For regular OFDMsymbols, some sub-carriers are used as pilot sub-carriers to carry pilotsymbols while the others are used as data sub-carriers to carry datasymbols. The pilot sub-carriers are modulated by pilot symbols generatedby QPSK. The data sub-carriers are modulated by complex data symbolsgenerated by QAM mapping. STTD coding is applied to the pilotsub-carrier pairs located at the same frequency within one STTD block.

Referring to FIG. 2, a block diagram of a Multiple-Input Multiple-Output(MIMO) Orthogonal Frequency Division Multiplexing (OFDM) transmitterprovided by an embodiment of the invention is shown. The OFDMtransmitter shown in FIG. 2 is a two-output OFDM transmitter, thoughmore generally there may be a plurality of M transmitting antennae. AnOFDM transmitter 10 takes binary data as input but data in other formsmay be accommodated. The binary data is passed to a coding/modulationprimitive 12 responsible for encoding, interleaving, and modulating thebinary data to generate data symbols, as is well known to those skilledin the art. The coding/modulation primitive 12 may include a number ofprocessing blocks, not shown in FIG. 2. An encoder 14 applies Space-TimeBlock Coding (SBTC) to the data symbols. The encoder 14 also separatesthe data symbols into a first processing path 16 and a second processingpath 18, by sending alternate data symbols along each of the twoprocessing paths. In the more general case in which the OFDM transmitter10 includes M transmitting antennae, the encoder 14 separates the datasymbols into M processing paths.

The data symbols sent along the first processing path 16 are sent to afirst OFDM component 20. The data symbols are first passed to ademultiplexer 22 in the first OFDM component 20, after which the datasymbols are treated as sub-carrier components. The data symbols are thensent to a pilot inserter 24, where pilot symbols are inserted among thedata symbols. Collectively, the data symbols and pilot symbols arereferred to hereinafter simply as symbols. The symbols are passed to anInverse Fast Fourier Transform

(IFFT) processor 26, then to a multiplexer 28 where they are recombinedinto a serial stream. A guard inserter 30 adds prefixes to the symbols.Finally, the OFDM signals are passed through a hard limiter 32, adigital-to-analog converter 34, and a radio frequency (RF) transmitter36 which transmits OFDM symbols as a signal through a first transmittingantenna 37. In most embodiments, each element in the first OFDMcomponent 20 is a processor, a component of a larger processor, or acollection of processors or any suitable combination of hardware,firmware and software. These might include general purpose processors,ASICs, FPGAs, DSPs to name a few examples.

The pilot inserter 24 is connected to receive space-time coded pilotsymbols from pilot STBC function 23 which performs STBC on pilot symbols21. The pilot STBC block 23 takes two pilot symbols at a time forexample P₁ and P₂ as indicated in FIG. 2 and generates an STBC blockconsisting of a two by two matrix having (P₁, P₂) in the first row andhaving (−P₂*, P₁*) in the second row. It is the first row of this STBCblock that is inserted by the pilot inserter 24.

The data symbols sent along the second processing path 18 are sent to asecond OFDM component 38 which includes processors similar to thoseincluded in the first OFDM component 20. However, the pilot inserter 40inserts encoded pilot symbols from the second row of the STBC blockproduced by the pilot STBC function 23. The symbols sent along thesecond processing path 18 are ultimately transmitted as a signal througha second transmitting antenna 42.

Referring now to FIG. 3, a block diagram of a MIMO-OFDM receiver isshown. An OFDM receiver 50 includes a first receiving antenna 52 and asecond receiving antenna 54 (although more generally there will be oneor more receiving antennae). The first receiving antenna 52 receives afirst received signal. The first received signal is a combination of thetwo signals transmitted by the two transmitting antennae 37 and 42 ofFIG. 2, although each of the two signals will have been altered by arespective channel between the respective transmitting antenna and thefirst receiving antenna 52. The second receiving antenna 54 receives asecond received signal. The second received signal is a combination ofthe two signals transmitted by the two transmitting antennae 37 and 42of FIG. 2, although each of the two signals will have been altered by arespective channel between the respective transmitting antenna and thesecond receiving antenna 54. The four channels (between each of the twotransmitting antennae and each of the two receiving antennae) may varywith time and with frequency, and will usually be different from eachother.

The OFDM receiver 50 includes a first OFDM component 56 and a secondOFDM component 58 (although in general there will be N OFDM components,one for each receiving antenna). The first OFDM component 56 includes aRF receiver 59, and an analog-to-digital converter 60, which convertsthe first received signal into digital signal samples. The signalsamples are passed to a frequency synchronizer 62 and a frequency offsetcorrector 64. The signal samples are also fed to a frame/timesynchronizer 66. Collectively, these three components producesynchronized signal samples.

The synchronized signal samples represent a time sequence of data. Thesynchronized signal samples are passed to a demultiplexer 68, thenpassed in parallel to a Fast Fourier Transform (FFT) processor 70. TheFFT processor 70 performs an FFT on the signal samples to generateestimated received symbols which are multiplexed in MUX 76 and sent asreceived symbols to decoder 78. Ideally, the received symbols would bethe same as the symbols fed into the IFFT processor 26 at the OFDMtransmitter 10. However, as the received signals will have likely beenaltered by the various propagation channels, the first OFDM component 56must correct the received symbols by taking into account the channels.The received symbols are passed to a channel estimator 72, whichanalyses received pilot symbols located at known times and frequencieswithin the OFDM frame. The channel estimator 72 compares the receivedpilot symbols with what the channel estimator 72 knows to be the valuesof the pilot symbols as transmitted by the OFDM transmitter 10, andgenerates an estimated channel response for each frequency and timewithin the OFDM symbol. The estimated channel responses are passed todecoder 78. The channel estimator 72 is described in detail below.

The second OFDM component 58 includes similar components as are includedin the first OFDM component 56, and processes the second received signalin the same manner as the first OFDM component 56 processes the firstreceived signal. Each OFDM component passes OFDM symbols to the decoder78.

The decoder 78 applies STBC decoding to the OFDM symbols, and passes thesymbols to a decoding/demodulating primitive 80 responsible fordecoding, de-interleaving, and demodulating the symbols to generateoutput binary data, as is well known to those skilled in the art. Thedecoding/demodulation primitive 80 which may include a number ofadditional processing blocks, not shown in FIG. 2. Each element in theOFDM components 56 and 58 is a processor, a component of a largerprocessor, or a collection of processors.

Referring now to FIG. 4, a method by which each of the pilot inserters24 and 40 of FIG. 2 inserts pilot symbols among the data symbols isshown. The method will be described with reference to the pilot inserter24 in the first OFDM component 20. At step 100, the pilot inserter 24receives data symbols from the demultiplexer 22. At step 102 the pilotSTBC function 23 generates (or receives) two pilot symbols. At step 104the pilot STBC function 23 applies STBC encoding to the pilot symbols,so as to generate an STBC block of encoded pilot symbols. The encodedpilot symbols generated for the first transmitting antenna 37 will beone row of the STBC block and will have a number equal to the number oftransmitting antennae in the OFDM transmitter. Thus, for a two antennasystem a 2×2 STBC block is generated.

At step 106 the pilot inserter 24 inserts the encoded pilot symbolswithin the OFDM symbol. Encoded pilot symbols are inserted in a diamondlattice pattern. The diamond lattice pattern uses the same frequenciesas the other diamond lattice patterns, but has a temporal offset fromthe other diamond lattice patterns. Preferably, the temporal offset foreach diamond lattice pattern is one symbol (in the time direction) fromanother diamond lattice pattern, so that the diamond lattice patternsuse consecutive symbols in the time direction of the OFDM frame.

The diamond lattice pattern in which each encoded pilot symbol isinserted within the OFDM frame is preferably a perfect diamond latticepattern. To achieve this, the encoded pilot symbol is inserted at eachof a first subset of frequencies. The frequencies within the firstsubset of frequencies are spaced equally apart by a pilot spacing. Theencoded pilot symbol is inserted at each of the first subset offrequencies for an STBC block (two OFDM symbols). At some later time,the encoded pilot symbols are inserted at each of a second subset offrequencies. The frequencies within the second subset of frequencies areshifted from the frequencies within the first subset of frequencies byhalf of the pilot spacing within the frequency direction. The pilotinserter 24 continues to insert encoded pilot symbols, alternatingbetween the first subset of frequencies and the second subset offrequencies.

Alternatively, a different pilot pattern can be used, as long as thesame pilot pattern is used for each of the at least one encoded pilotsymbols unique to the transmitting antenna 37, and as long as the pilotpatterns for the encoded pilot symbols are offset from each other in thetime direction of the OFDM frame. For example, a regular diagonallattice pattern may be used, the diamond shaped lattice being a specialcase of this.

The pilot inserter 40 inserts pilot symbols using the same method,although the pilot symbols will be the other half of the STBC block 42.The encoded pilot symbols unique to the second transmitting antenna 42are inserted in the OFDM frame at the same symbol locations at which theencoded pilot symbols corresponding to the first transmitting antenna 37are inserted.

Referring to FIG. 5, an example pilot pattern generated using the methodof FIG. 4 is shown. Pilot and data symbols are spread over the OFDMframe in a time direction 120 and a frequency direction 122. Mostsymbols within the OFDM frame are data symbols 124. A first set ofencoded pilot symbols 126 corresponding to the first transmittingantenna 37 is inserted in a diamond lattice pattern. A second set ofencoded pilot symbols 128 corresponding to the first transmittingantenna 37 is inserted in a diamond lattice structure at the samefrequencies as the first set of encoded pilot symbols, but offset by oneOFDM symbol location in the time direction 120. In the illustratedexample two of every four OFDM symbols carry encoded pilot symbols. Eachother transmitting antenna transmits using the same pattern. The pairsof consecutive pilot symbols on a sub-carrier consist of two raw pilotsymbols STBC encoded. The same pattern is transmitted by the secondantenna.

The power of the encoded pilot symbols 126, 128 may be increasedcompared to the traffic data symbol 124. The power increase of theencoded pilot can be dynamically adjusted with respect to thetransmitting data symbol power level or modulation type (QAM size), oras a function of channel quality. The location of diamond latticepattern may also be optimized to allow a fast extraction of scatteredpilot without using the computing. This may be achieved if the pilotsubcarriers are spaced in the frequency direction by 2̂n. In the multiplebase station transmission arrangement, the location of the diamondlattice pattern can be cyclic offset both in time direction and infrequency direction amongst adjacent base stations to form a diamondlattice re-use pattern.

Referring now to FIGS. 6 to 10, a channel estimation method is describedwhich is based on the pilot insertion method above. This inventionpresents a simple 2-dimensional channel interpolator for MIMO-OFDMsystem with low pilot density for fast fading channels both in time andin frequency. The goal of channel estimation is to estimate the channelcharacteristics for each sub-carrier and at each time for each possibletransmit antenna, receive antenna combination. Referring to FIG. 6, forthe two transmit antenna, two receive antenna example, shown are twotransmit antennas T×1 140 and T×2 142 and two receive antennas R×1 144and R×2 146. Channel estimation estimates a channel for each sub-carrierand at each time between T×1 140 and R×1 144 indicated as each H₁₁ 148,a channel between T×1 140 and R×2 146 indicated by transfer function H₁₂150, a channel estimate for transmitter T×2 142 to R×1 144 indicated astransfer function H₂₂ 152 and finally, a channel estimate fortransmitter T×2 142 to receiver R×2 146 indicated as transfer functionH₂₁ 154.

Some advantages for the proposed method compared to some existingmethods are: (1) robust to high mobility-speed (2) a reduction of thescattered pilot grid density and therefore a reduction of the pilotoverhead.

Let P₁ and P₂ be the two pilot symbols encoded in an STBC block andtransmitted by two antennas on one sub-carrier in consecutive OFDMsymbols. Then at the first receive antenna, the following relationshipexists for each sub-carrier on which pilot symbols are transmitted,where it is assumed the channel response H_(ij) is constant over twoOFDM frames:

$\begin{bmatrix}Y_{1,1} \\Y_{1,2}\end{bmatrix} = {\begin{bmatrix}P_{1} & P_{2} \\{- P_{2}^{*}} & P_{1}^{*}\end{bmatrix}\begin{bmatrix}H_{11} \\H_{21}\end{bmatrix}}$

Y_(1,1) is the received data on the first antenna on the sub-carrier inthe first of the two consecutive OFDM symbols, and Y_(1,2) is thereceived data on the first antenna on the sub-carrier in the second ofthe two consecutive symbols. This can be solved for H₁₁, H₂₁ to yield:

$\begin{bmatrix}H_{11} \\H_{21}\end{bmatrix} = {{\frac{1}{{P_{1}}^{2} + {P_{2}}^{2}}\begin{bmatrix}P_{1}^{*} & {- P_{2}} \\P_{2}^{*} & P_{1}\end{bmatrix}}\begin{bmatrix}Y_{1,1} \\Y_{1,2}\end{bmatrix}}$

A similar process for the second antenna yields

$\begin{bmatrix}H_{12} \\H_{22}\end{bmatrix} = {{\frac{1}{{P_{1}}^{2} + {P_{2}}^{2}}\begin{bmatrix}P_{1}^{*} & {- P_{2}} \\P_{2}^{*} & P_{1}\end{bmatrix}}\begin{bmatrix}Y_{2,1} \\Y_{2,2}\end{bmatrix}}$

where Y_(2,1) is the received data on the second antenna on thesub-carrier in the first of the two consecutive OFDM symbols, andY_(2,2) is the received data on the second antenna on the sub-carrier inthe second of the two consecutive OFDM symbols.

Using this technique, a channel estimate is made for each pilotsub-carrier, and for each pair of OFDM symbols used to transmit STBCblocks.

For the example of FIG. 5, the result is a channel estimate, for each ofthe possible channels (these are for channels in this example as shownin FIG. 6) for each pair of pilot symbols transmitted. This isillustrated in FIG. 7 where only sub-carriers used to transmit pilotsare shown. A channel estimate 150 is generated for each pair of(consecutive in time) OFDM frames for each pilot sub-carrier. Thisresults in channel estimates 150, 152, 154 for the first and secondframes, and channel estimates 156, 158, 160 for the fifth and sixthframes and so on.

The channel estimates are made on a STBC block by block basis so thatthe pattern of channel estimate shown in FIG. 7 develops over time. Thenext step in the process is to perform an interpolation based on thechannel estimate of FIG. 7 to obtain channel estimates for the places inFIG. 7 which do not represent pilot channel positions. The manner inwhich this is done will be described for a single example, namely theunknown channel estimate indicated at 163 of FIG. 7. Channel estimatesare buffered on an ongoing basis and when the four channel estimates152, 156, 158 and 164 forming a diamond 162 surrounding the unknownchannel estimate 163 have been computed, it is time to interpolate toobtain a channel estimate for the unknown point 163. The channeltransfer function at the sub-carrier located at the centre of thediamond can be obtained from a simple 4 points two-dimensionalinterpolator. Three points two-dimensional interpolators can be used toobtain the channel estimates corresponding to the first or last usefulsub-carrier:

${H_{new}( {{n + 1},k} )} = {\frac{1}{4}( {{H( {n,k} )} + {H( {{n + 2},k} )} + {H( {{n + 1},{k - 1}} )} + {H( {{n + 1},{k + 1}} )}} )}$

where (k=2, . . . , N_(pilot)−1)

${H_{new}( {{n + 1},1} )} = {\frac{1}{4}( {{H( {n,1} )} + {H( {{n + 2},1} )} + {2{H( {{n + 1},2} )}}} )}$${H_{new}( {{n + 1},N_{pilot}} )} = {\frac{1}{4}( {{H( {n,N_{pilot}} )} + {H( {{n + 2},N_{pilot}} )} + {2{H( {n,{N_{pilot} - 1}} )}}} )}$

where k is the pilot sub-carrier index, n is the channel estimate index(or STBC block number—one channel estimate per sub-carrier for every twosymbols), and N_(pilot) is the number of pilot sub-carriers (6 in theexample of FIG. 7). H_(new) is the newly interpolated channel estimatefor the i^(th) channel estimation period, and the j^(th) pilotsub-carrier. H(i, j) is the channel estimate determined as describedpreviously from the pilot symbols. A three points interpolator wouldalso be performed for the last STBC blocks in the OFDM frame (i.e. thelast two OFDM symbols).

These calculations are done for each transmit antenna, receiver antennacombination. It is noted that this is just one example of how thechannel estimates can be interpolated.

If the original distance between pilot sub-carriers in the frequencydirection is D_(f), after first step of interpolation described above,the pilot sub-carriers' separation becomes

$\frac{D_{f}}{2}.$

In some embodiments, to remove noise, the channel estimates thuscomputed are filtered at each channel estimation period. This is shownin FIG. 6 where the channel estimates 170 for one channel estimationperiod are shown entering filter 172 to produce filtered channelestimates. For example, a simple 3 point moving iterative smoothingalgorithm may be applied to H′:

${H_{sm}^{\prime}( {n,k} )} = {{H_{sm}^{\prime}( {n,{k - 1}} )} + {\frac{1}{3}( {{H^{\prime}( {n,{k + 1}} )} + {H_{sm}^{\prime}( {n,{k - 2}} )}} )}}$

where k=3, . . . , 2 N_(pilot)−2. It is to be understood that otherfiltering algorithms may be employed.

After the interpolation of the pilot channel estimate as summarized inFIG. 7, there will be a channel estimate for each sub-carrier on whichpilot channel information was transmitted and for each two OFDM symbolperiod over which pilot channelling information was transmitted.Referring to FIG. 5, this means that there will be a channel estimatefor each antenna for time frequency points which are shaded to indicatethat pilot channel information was transmitted. There will also bechannel estimates for the time frequency point in the centre of thediamond shaped lattice structure of FIG. 7. However, for points whichare not pilot symbol transmission time-frequency points nor points whichare at the centre of a diamond shaped lattice of such points, there willbe no channel estimate yet computed. The next step is to perform afurther interpolation step to develop channel estimates for these otherpoints.

In some embodiments, Cubic Lagrange interpolation and linearinterpolation (for the sub-carriers near the first and the last usefulsub-carrier) in the frequency direction are used to obtain the channeltransfer function at all sub-carriers for each STBC block (for each pairof OFDM symbols).

The coefficients of the Cubic Lagrange interpolator can be calculated as

${{\mu (i)} = {{\frac{i}{D_{f}/2}\mspace{14mu} i} = 1}},2,\ldots \mspace{14mu},\frac{D_{f}}{2}$${q_{- 1}(\mu)} = {{{- \frac{1}{6}}\mu^{3}} + {\frac{1}{2}\mu^{2}} - {\frac{1}{3}\mu}}$${q_{0}(\mu)} = {{\frac{1}{2}\mu^{3}} - \mu^{2} - {\frac{1}{2}\mu} + 1}$${q_{1}(\mu)} = {{{- \frac{1}{2}}\mu^{3}} + {\frac{1}{2}\mu^{2}} + \mu}$${q_{2}(\mu)} = {{{- \frac{1}{6}}\mu^{3}} - {\frac{1}{6}\mu}}$

The channel transfer functions at data sub-carriers are given by

${H_{interp}( {{( {j - 1} ) \cdot \frac{D_{f}}{2}} + i} )} = {\sum\limits_{n = {- 1}}^{2}{{q_{n}( {\mu (i)} )} \cdot {H_{sm}^{\prime}( {j + n} )}}}$

where j=2, . . . , N_(pilot)−2.

This is illustrated in FIG. 9 where the estimated channel responses arefed to the Legrange cubic interpolator function 175 which outputs valuesfor all intermediate sub-carriers. Other interpolations mayalternatively be employed.

In some embodiments, every OFDM symbol contains some pilot insertionpoints and as such this completes the interpolation process. In otherembodiments, there are some OFDM symbols which do not have any pilotinsertion points. To get channel estimates for these OFDM symbols, aninterpolation in time of the previously computed channel estimates isperformed. In high mobility applications, pilots should be included inevery OFDM symbol avoiding the need for this last interpolation in timestep.

FIG. 10 presents an overall block diagram of the interpolation methodproposed for two transmit antennas. An example set of performanceresults for the proposed MIMO-OFDM channel estimation algorithm is shownin FIG. 10. The performance of the 2-D channel estimation algorithm isclose to the performance of ideal channel (only 0.5 dB loss) at veryhigh Doppler spread.

Referring now to FIGS. 10 and 3, the channel estimation method iscarried out by the channel estimator 72 in order to estimate a channelresponse for each sub-carrier and each OFDM symbol within an OFDM frame.The channel estimation method starts at step 500 by extracting the pilotsymbols in the frequency domain for each receive antenna. This isfollowed by a channel response matrix computing step 502; whereby thereceived signal received by the receiving antenna is decoded, which ineffect performs a time average of the encoded pilot symbols at eachpoint in the pilot pattern. For example, suppose the receiving antennareceives an OFDM frame having a pilot pattern as shown in FIG. 5(although the symbol 126 will now be a linear combination of the encodedpilot symbol transmitted at this location by each of the transmittingantenna, and the symbol 128 will be a linear combination of the encodedpilot symbol transmitted at this location by each of the transmittingantenna). Following decoding, the pilot symbol at symbol location 126will be an average of the pilot symbol received at symbol location 126and the pilot symbol received at symbol location 128. The time averagingeffect produced by the STBC decoding, during step 503, can be viewed asa pre-processing step, as can steps 500 and 502. The actual channelestimation method can be described broadly in four steps. Following step503, during step 504 the channel estimator 72 estimates the channelresponse for each of a plurality of pilot symbols. For a diamond latticepattern, the plurality of pilot symbols will be four pilot symbolsforming a single diamond pattern. The channel estimator 72 estimates thechannel response of a central symbol, the central symbol having a timedirection value and a frequency direction value bounded by the timedirection values and the frequency direction values of the plurality ofpilot symbols. The central symbol preferably has a frequency directionvalue equal to the frequency direction values of two of the plurality ofpilot symbols, and has a time direction value midway between the timedirection values of the two pilot symbols having the same frequencydirection value as the central symbol. This can generally be describedas a four-point 2-D interpolation of the channel response between pilotsymbols. Third, the channel estimator 72 smoothes the channel responses(corresponding to both encoded pilot symbols and to the central symbol)in the frequency direction, preferably by performing a three-pointsmoothing, as per step 505. Fourth, the channel estimator 72 performs aninterpolation in the frequency direction to estimate the channelresponse for remaining symbols, as per step 506. The interpolation maybe a linear interpolation for symbols having a frequency direction valueequal to a first or a last useful sub-carrier within the OFDM symbol,and a cubic Lagrange interpolation otherwise.

The method of inserting pilot symbols (described above with reference toFIG. 4) and the channel estimation method (described above withreference to FIG. 10) need not be used together. Any channel estimationmethod may be used by the OFDM receiver to estimate the channelresponses for an OFDM frame containing encoded pilot symbols insertedusing the method described above. However, due to the sparsedistribution of the pilot symbols in the pilot pattern described abovewith reference to FIG. 4 and FIG. 5, a two-dimensional interpolationmethod is preferable over a one-dimensional interpolation method.Similarly, the channel estimation method may be applied to an OFDM framecontaining any pattern of pilot symbols.

The invention has been described with respect to a MIMO-OFDMcommunication system. The invention may also be used with advantage in asingle input-multiple output OFDM communication system, as the method ofinserting pilot symbols (described with reference to FIG. 4) and thechannel estimation method (described with reference to FIG. 10) do notdepend on the number of receiving antenna. Each receiving antenna withinthe OFDM receiver 50 performs channel estimation independently,regardless of the number of receiving antennae present.

The channel estimation method described with reference to FIG. 10 willalso be advantageous in an OFDM communication system having only onetransmitting antenna, as the method provides an improved interpolationof the channel response regardless of the number of transmittingantenna. The method of inserting pilot symbols described with referenceto FIG. 11 may be used in an OFDM communication system having only onetransmitting antenna, but will not be as advantageous as in an OFDMcommunication system having more than one transmitting antenna as therewill be no reduction in overhead.

The method of inserting pilot symbols and the channel estimation methodare preferably implemented on the OFDM transmitter and on the OFDMreceiver respectively in the form of software instructions readable by adigital signal processor. Alternatively, the methods may be implementedas logic circuitry within an integrated circuit. More generally, anycomputing apparatus containing logic for executing the describedfunctionality may implement the methods. The computing apparatus whichimplements the methods (in particular the pilot inserter or the channelestimator) may be a single processor, more than one processor, or acomponent of a larger processor. The logic may comprise externalinstructions stored on a computer-readable medium, or may compriseinternal circuitry.

FIGS. 12-17 illustrate various embodiments of the present invention.These embodiments provide schemes for data mapping on downlink (DL) anduplink (UL) transmission antennas. A DL transmission is a transmissionover a transmission resource from a base station to one or more mobilestations. A UL transmission is a transmission over a transmissionresource from one or more mobile stations to a base station. In someembodiments, a transmission resource is a plurality of OFDM symbolstransmitted on a plurality of OFDM subcarriers. In some embodiments asignal processing scheme is used that supports high data rates at verylow packet and delay losses, also known as latencies, over a distributedIP wireless network. Transmissions that have low-latency enablereal-time mobile interactive and multimedia applications. In someembodiments, the signal processing scheme delivers higher qualitywireless service and improved cost effectiveness over current wirelessdata technologies.

In some embodiments the schemes are used for multiple input multipleoutput (MIMO) OFDM transmission.

Inserting Data Symbols for DL MIMO

FIG. 12A illustrates first and second time-frequency patterns 1000,1001for mapping DL data symbols for transmission on two antennas using aMIMO OFDM scheme. For example, pilot pattern 1000 is for transmission ona first antenna and pilot pattern 1001 is for transmission on a secondantenna.

The time-frequency patterns 1000,1001 are two dimensional plots in whichone dimension is a time direction and the other dimension is a frequencydirection. In the frequency direction, each discrete horizontal rowrepresents a single sub-carrier. Each discrete vertical columnrepresents an OFDM symbol. The time-frequency patterns 1000,1001 areshown to be two dimensional plots that are nine sub-carriers by ten OFDMsymbols in size.

Employing a particular symbol-by-symbol frequency hopping sequence, datais mapped onto the time-frequency patterns 1000,1001. In aspread-spectrum communications system, the frequency hopping sequenceallows for the transmission to move or “hop” among numerous frequenciesmany times per second. As an example, the transmission may hop among 128frequencies 1,024 times per second.

Frequency hopping is a variant of spread spectrum that uses a techniquethat enables coexistence of multiple networks (or other devices) in asame area. An example of frequency hopping is IEEE 802.11 FrequencyHopping PHY, which uses 79 non-overlapping frequency channels with 1 MHzchannel spacing. In some implementations frequency hopping enablesoperation of up to 26 collocated networks, enabling high aggregatethroughput. Frequency hopping is resistant to multi-path fading throughthe inherent frequency diversity mechanism.

An OFDM data symbol stream 1010 is divided into a first data symbolstream 1012 and a second data symbol stream 1014. The first and seconddata symbol streams 1012,1014 are mapped onto the first and secondtime-frequency patterns 1000,1001 respectively with an identical hoppingsequence. The data symbol stream 1010, which includes data symbols “S₂₀,. . . S₂, S₁”, is mapped as follows: odd-numbered symbols of data symbolstream 1010, that is “S₁₉ . . . S₃, S₁”, are mapped to time-frequencypattern 1000 to be transmitted by a first antenna and even-numbered datasymbols of data symbol stream 1010, that is “S₂₀ . . . S₄,S₂”, aremapped to time-frequency pattern 1001 to be transmitted by a secondantenna. Odd-numbered symbols in the first time-frequency pattern 1000are plotted in corresponding locations to even-numbered symbols on thesecond time-frequency pattern 1001. For example, odd-numbered symbol S₁in the first time-frequency pattern 1000 is located at the same locationin time-frequency pattern 1000 as even-numbered symbol S₂ in the secondtime-frequency pattern 1001. Other pairs of odd/even-numbered datasymbols are similarly distributed throughout the time-frequency patterns1000 and 1001 according to the hopping sequence.

FIG. 12A illustrates data symbol stream 1010 divided into odd andeven-numbered symbols, but more generally the data symbol stream 1010can be divided in any manner that results in a first data symbol streamand a second data symbol stream. For example, the data symbol stream maybe divided into first and second data symbol streams of pairs ofsymbols, as opposed to dividing the data stream into individual odd andeven symbols on first and second respective data symbol streams.

In some embodiments, a frequency hopping pattern for data symbols, inwhich the frequency of a data symbol stream hops every OFDM symbol, iscreated by inserting one or more data symbols from the data symbolstream for transmission to one or more mobile stations on one or moresubcarriers of an allocated transmission bandwidth that are alldifferent from the subcarriers used by the same data symbol stream fortransmission to a same one or more mobile stations in a previous hop.This is the case for the example of FIG. 12A where during the sixth OFDMsymbol for example, S₁₇ and S₁₉ of data stream 1012 are transmitted onthe second and ninth subcarriers. During the seventh OFDM symbol, S₁₃and S₁₅ are transmitted on the fifth and seventh subcarriers, none ofwhich are the same as were used during the sixth OFDM symbol.

The illustrative example of FIG. 12A shows frequency hopping for eachOFDM symbol duration. In some embodiments frequency hopping occurs aftera group of OFDM symbols, where a group is at least two symbols.

One of ordinary skill in the art will recognize that FIG. 12A shows only20 data symbols in the data stream 1010 for illustrative purposes only.Depending on the size of the data symbol stream, the time-frequencypatterns 1000,1001 can include additional or fewer data symbols thanthat shown in FIG. 12A. Additionally, the number of transmissionantennas is not limited to two antennas as described above; more thantwo antennas can be employed for transmission of data.

The data symbols are modulated onto a specific hopping sequence fortransmission to at least one mobile station. Different transmissionfrequencies or subcarriers are utilized in each OFDM symbol fortransmission to each mobile station. The two time-frequency patterns1000, 1001 of FIG. 12A illustrate the location of data symbols “. . .S₂₀, . . . , S₂, S₁” 1010 transmitted on two respective antennas.Another set of data symbols occupying locations in time-frequencycorresponding to symbol locations in the time-frequency patterns1000,1001 is indicated generally by the cross hatch pattern identifiedby reference character 1030. The set of data symbols 1030 may betransmitted by one or more antenna that are different than the tworespective antennas transmitting time-frequency patterns 1000,1001. Insome embodiments, the set of data symbols 1030 is transmitted byantennas belonging to the same base station as the two antennas used totransmit time-frequency patterns 1000,1001. In some embodiments the setof data symbols 1030 represents data symbols transmitted by one or moreantennas of a different base station than the base station including thetwo antennas used to transmit time-frequency patterns 1000,1001.However, the base station used to transmit time-frequency patterns1000,1001 does not insert data symbols at the locations of the set ofdata symbols 1030 to avoid interference.

Mobile stations receiving data and pilot symbols know where data andpilot symbols are located in time-frequency based on a particularfrequency-hopping pattern for data and particular pilot pattern of whichthey are aware. In some embodiments, the base station indicates to themobile station a particular data symbol frequency-hopping pattern andpilot pattern via a control signaling channel. In some embodiments, themobile station indicates to the base station a particular data symbolfrequency hopping pattern and pilot pattern to be used by the basestation.

In some embodiments a hopping sequence is periodic having a perioddefined by a “super slot”, generally indicated at 1050. The number ofOFDM symbols defines the size of the super slot in the particularfrequency hopping sequence. The super slot 1050 of FIG. 12A is shown tohave a duration of ten OFDM symbols. More generally, it is to beunderstood that the duration of the super slot is implementationspecific and can be greater than or less than the ten OFDM symbols shownin FIG. 12A. Similarly, the number of subcarriers in the time-frequencypatterns 1000,1001 is implementation specific and can be greater than orless than the nine subcarriers that are shown in the illustrated exampleof FIG. 12A.

In some embodiments, subcarriers in the frequency direction are a set ofconsecutive subcarriers of an allocated frequency band. In someembodiments, subcarriers in the frequency direction are a set ofsubcarriers that are not necessarily consecutive, but are a selection ofsubcarriers grouped together to form a “logical” subband of subcarriers.

In FIG. 12A, the data symbols from data streams 1012,1014 are insertedin the respective time-frequency patterns 1000,1001 by using an OFDMsymbol by OFDM symbol arrangement. For example, in time-frequencypattern 1000, data symbols s₁, s₃ and s₅ are inserted in the tenth OFDMsymbol, then data symbol s₇ is inserted in the ninth OFDM symbol, thendata symbols s₉ and s₁₁ are inserted in the eighth OFDM symbol, thendata symbols s₁₃ and s₁₅ are inserted in the seventh OFDM symbol, andthen data symbols s₁₇ and s₁₉ are inserted in the sixth OFDM symbol.This is a particular manner for inserting data symbols in thetime-frequency patterns, but those skilled in the art will realize thatthere are various ways that the data symbols could be inserted in thetime-frequency patterns. For example, in some embodiments, the datasymbols are inserted using a subcarrier by subcarrier arrangement. Forexample, data symbols are inserted on particular OFDM symbol durationsfor a first subcarrier, then data symbols are inserted on particularOFDM symbol durations for a second subcarrier, and so on.

FIG. 12A shows only a portion of the time-frequency patterns 1000,1001filled with symbols to be transmitted from a single data symbol stream1010. It is to be understood that data symbols can fill some or all ofthe available symbol slots in the time-frequency pattern after pilotsymbols are allocated in the respective time-frequency pattern. Forexample, there may be multiple data symbol streams in which each streamis divided into a first data symbol stream and a second data symbolstream, and these streams are inserted in locations in thetime-frequency patterns 1000,1001 with a given respective hoppingsequence. While only a single data symbol stream is shown in FIG. 12A,it is to be understood that the time-frequency patterns can be used forany number of data symbol streams equal to or greater than the one datasymbol stream.

In addition to the time-frequency patterns 1000,1001 being used fordata, scattered pilot symbols are also transmitted within the same OFDMresource, in a manner that does not conflict with the data transmission.In the illustrated example of FIG. 12A, pilot symbols in eachtime-frequency pattern 1000,1001 include diagonal arrangements 1020,1025of alternating pilot symbols 1026,1028 and null symbol locations 1027.The pilot symbols 1026 in time-frequency pattern 1000 are located wherenull symbol locations 1027 occur in time-frequency pattern 1001 andpilot symbols 1028 in time-frequency pattern 1001 are located where nullsymbol locations 1027 occur in time-frequency pattern 1000. Pilot symbolpatterns for use with MIMO OFDM DL time-frequency patterns will bedescribed in further detail below.

In a particular example of mapping data symbols using afrequency-hopping scheme as described above, pairs of odd and even datasymbols are mapped to a set of twenty-four respective subcarriers in atime-frequency pattern for each of two respective antennas. Using aspace-time transmit diversity (STTD) coding scheme the mapping of datasymbols is as follows:

TABLE 1 STTD coding of data symbols for two antennas Antenna 1 Antenna 2Even Odd Even Odd Subcarrier Symbol Symbol Symbol Symbol Subcarrier 0 s₀−s₂₄* s₂₄ s₀* Subcarrier 1 s₁ −s₂₅* s₂₅ s₁* Subcarrier 2 s₂ −s₂₆* s₂₆s₂* Subcarrier 3 s₃ −s₂₇* s₂₇ s₃* Subcarrier 4 s₄ −s₂₈* s₂₈ s₄*Subcarrier 5 s₅ −s₂₉* s₂₉ s₅* Subcarrier 6 s₆ −s₃₀* s₃₀ s₆* Subcarrier 7s₇ −s₃₁* s₃₁ s₇* Subcarrier 8 s₈ −s₃₂* s₃₂ s₈* Subcarrier 9 s₉ −s₃₃* s₃₃s₉* Subcarrier 10 s₁₀ −s₃₄* s₃₄ s₁₀* Subcarrier 11 s₁₁ −s₃₅* s₃₅ s₁₁*Subcarrier 12 s₁₂ −s₃₆* s₃₆ s₁₂* Subcarrier 13 s₁₃ −s₃₇* s₃₇ s₁₃*Subcarrier 14 s₁₄ −s₃₈* s₃₈ s₁₄* Subcarrier 15 s₁₅ −s₃₉* s₃₉ s₁₅*Subcarrier 16 s₁₆ −s₄₀* s₄₀ s₁₆* Subcarrier 17 s₁₇ −s₄₁* s₄₁ s₁₇*Subcarrier 18 s₁₈ −s₄₂* s₄₂ s₁₈* Subcarrier 19 s₁₉ −s₄₃* s₄₃ s₁₉*Subcarrier 20 s₂₀ −s₄₄* s₄₄ s₂₀* Subcarrier 21 s₂₁ −s₄₅* s₄₅ s₂₁*Subcarrier 22 s₂₂ −s₄₆* s₄₆ s₂₂* Subcarrier 23 s₂₃ −s₄₇* s₄₇ s₂₃*

The subcarriers 0-23 in Table 1 above are numbered consecutively, butwhen the data symbols associated with these respective subcarriers areinserted in time-frequency patterns to be transmitted by two respectiveantennas, the subcarriers 0-23 correspond to subcarriers allocated tothe time-frequency patterns over multiple OFDM symbol durations. FIG.12B shows illustrates an example of a mapping of the STTD coded datasymbols of Table 1 on time-frequency patterns 1070,1071. Time-frequencypatterns 1070,1071 have a similar structure to FIG. 12A, but use elevensubcarriers over ten OFDM symbol durations. In time-frequency pattern1070, the data symbols from subcarriers 0-3 of Table 1 are inserted onsubcarriers one, three, five and nine, the data symbols from subcarriers4-8 of Table 1 are inserted on subcarriers four, six, seven, ten andeleven, and so on. Inserting data symbols in time-frequency pattern 1071is performed in a similar manner.

In a particular example of mapping data symbols using afrequency-hopping scheme described above, pairs of odd and even datasymbols are mapped to a set of twenty-four respective subcarriers in atime-frequency pattern for each of two respective antennas. For aspatial multiplexing (SM) coding scheme the mapping of data symbols isas follows:

TABLE 2 SM coding of data symbols for two antennas Antenna 1 Antenna 2Even Odd Even Odd Subcarrier Symbol Symbol Symbol Symbol Subcarrier 0 s₀s₄₈ s₁ s₄₉ Subcarrier 1 s₂ s₅₀ s₃ s₅₁ Subcarrier 2 s₄ s₅₂ s₅ s₅₃Subcarrier 3 s₆ s₅₄ s₇ s₅₅ Subcarrier 4 s₈ s₅₆ s₉ s₅₇ Subcarrier 5 s₁₀s₅₈ s₁₁ s₅₉ Subcarrier 6 s₁₂ s₆₀ s₁₃ s₆₁ Subcarrier 7 s₁₄ s₆₂ s₁₅ s₆₃Subcarrier 8 s₁₆ s₆₄ s₁₇ s₆₅ Subcarrier 9 s₁₈ s₆₆ s₁₉ s₆₇ Subcarrier 10s₂₀ s₆₈ s₂₁ s₆₉ Subcarrier 11 s₂₂ s₇₀ s₂₃ s₇₁ Subcarrier 12 s₂₄ s₇₂ s₂₅s₇₃ Subcarrier 13 s₂₆ s₇₄ s₂₇ s₇₅ Subcarrier 14 s₂₈ s₇₆ s₂₉ s₇₇Subcarrier 15 s₃₀ s₇₈ s₃₁ s₇₉ Subcarrier 16 s₃₂ s₈₀ s₃₃ s₈₁ Subcarrier17 s₃₄ s₈₂ s₃₅ s₈₃ Subcarrier 18 s₃₆ s₈₄ s₃₇ s₈₅ Subcarrier 19 s₃₈ s₈₆s₃₉ s₈₇ Subcarrier 20 s₄₀ s₈₈ s₄₁ s₈₉ Subcarrier 21 s₄₂ s₉₀ s₄₃ s₉₁Subcarrier 22 s₄₄ s₉₂ s₄₅ s₉₃ Subcarrier 23 s₄₆ s₉₄ s₄₇ s₉₅

Inserting Pilot Symbols for DL MIMO

FIG. 13A illustrates an example of first and second pilot patterns1200,1201 for DL transmission for a MIMO OFDM case for transmission ontwo antennas. For example, pilot pattern 1200 is for transmission on afirst antenna and pilot pattern 1201 is for transmission on a secondantenna.

The pilot patterns 1200,1201 are each two dimensional plots in which onedimension is a time direction and the other dimension is a frequencydirection. The pilot patterns 1200,1201 are shown to have a same size,in terms of a number of subcarriers and OFDM symbols, as thetime-frequency plot 1000,1001 in FIG. 13A.

Pilot symbols in the first pilot pattern 1200 are generally indicated byreference character 1210. Pilot symbols in the second pilot pattern 1201are generally indicated by reference character 1220. Null symbollocations in the first and second pilot patterns 1200,1201 are generallyindicated by reference character 1206.

The pilot symbols 1210 and null symbol locations 1206 in the first pilotpattern 1200 and pilot symbols 1220 and null symbol locations 1206 inthe second pilot pattern 1201 are illustrated in FIG. 13A occupyingsymbol locations in two negative sloping diagonal lines, respectively.The pilot symbols and null symbol locations in the illustrated exampleare located in symbol locations that are empty in the time-frequencypatterns 1000,1001 of FIG. 12A.

Null symbol locations 1206 represent an absence of a symbol in thatlocation of a given pilot pattern. In FIG. 13A, pilot symbols 1210 inthe first pilot pattern 1200 are alternated on the diagonal slope withnull symbol locations 1206. Pilot symbols 1220 in the second pilotpattern 1201 are alternated on the diagonal slope with null symbollocations 1206. The null symbol locations 1206 in the first pilotpattern 1200 are the locations of the pilot symbols 1220 in the secondpilot pattern 1201 and the null symbol locations 1206 in the secondpilot pattern 1201 are the locations of the pilot symbols 1210 in thefirst pilot pattern 1200. While the patterns 1200,1201 illustratealternating pilot symbols and null symbol locations in FIG. 13A, thoseskilled in the art will recognize that other combinations of null symbollocations and pilot symbols may be employed. For example, the nullsymbol locations and pilot symbols may be ordered in a repeating patternof two consecutive symbols of the diagonal slope being pilot symbolsfollowed by two consecutive symbols of the diagonal slope being nullsymbol locations.

In some embodiments, the pilot symbols 1210,1220 and null symbollocations 1206 are inserted as one or more positive sloping diagonallines. Furthermore, it is to be understood that pilot symbols can beallocated in patterns having different “rise-over-run” values for theslope, which is the relationship in the time (run) and frequency (rise)directions. FIG. 13A shows diagonal lines for each pilot pattern1200,1201 with a rise over run of −1. More generally, the slope of thepilot symbols is implementation specific and can vary from a largepositive value to a large negative value, excluding zero, allowing for adesirable number of pilots in a time-frequency pattern of a given size.In some implementations the pilot pattern is allocated prior to the datasymbols being allocated.

While one or more diagonal lines may have a periodic repetition ofdiagonal sloping lines, it is to be understood that the presentinvention is not to be limited to only periodic repetition of diagonalsloping lines. For example, the spacing between diagonal lines may notbe periodic in nature.

In some embodiments, different pilot patterns are defined that have adifferent spacing between parallel diagonal lines and one of thepatterns thus defined is selected for each base station having one ormore antennas. Having a different spacing between the parallel diagonallines, in a direction normal to the diagonal lines, enables the densityof the pilot symbols in a given time-frequency pattern to be increasedor decreased. For example, spacing diagonal lines closer togetherenables insertion of more pilots per super slot. In some embodiments thedensity of pilot symbols transmitted from can be varied from one superslot to another for the same base station by varying the spacing of theparallel diagonal lines in respective super slots.

In the example of FIG. 13A the pilot symbols and the null symbollocations in each of the two pilot patterns 1200,1201 are in respectivecollinear diagonal lines. In some embodiments, as opposed to alternatingpilot symbols and null symbol locations in the same collinear diagonalline, the pilot symbols and the null symbol locations form pairs ofparallel diagonal lines in which a first diagonal line of the pair isall pilot symbols and a second diagonal line is all null symbollocations. FIG. 13B illustrates a pair of pilot patterns 1240,1241having such a pattern. By way of example these pilot patterns have asame size as the pilot patterns 1200,1201 of FIG. 13A. However, this isnot intended to limit the scope of the invention to a time-frequencypattern having only one particular size. The size of the time-frequencypattern including both data and pilot symbols is implementationspecific. In pilot pattern 1240, diagonal lines 1251 have null symbollocations and diagonal lines 1250 have pilot symbols. In pilot pattern1241, diagonal lines 1261 have pilot symbols and diagonal lines 1260have null symbol locations. In some embodiments the pilot symbol patternis repeated for each super slot. In some embodiments the pilot symbolpattern is different for each super slot.

In some embodiments of the invention, additional pilot symbols areinserted in at least one time-frequency pattern of a group of thetime-frequency patterns to enable a higher density of pilot symbols forthat time-frequency pattern than for other time-frequency patterns ofthe group.

FIG. 13C illustrates a pair of pilot patterns 1270,1271. By way ofexample these pilot patterns have a same size as the pilot patterns1200,1201 of FIG. 13A. Pilot pattern 1270 has a higher density of pilotsymbols that pilot pattern 1271. Pilot pattern 1270 has three diagonallines 1280,1281 of pilot patterns. Pilot pattern 1271 has two diagonallines 1285 of pilot patterns. Two diagonal lines 1280 of the threediagonal lines in pilot pattern 1270 and the two diagonal lines 1285 inpilot pattern 1271 are similar to FIG. 13A in that the diagonal lines ofrespective patterns alternate between pilot symbols and null symbollocations. A third diagonal line 1281 of the three diagonal lines inpilot pattern 1270 is all pilot symbols. In some embodiments, locationsin pilot pattern 1271 corresponding to the location of the thirddiagonal line 1281 in pilot pattern 1270 are null symbol locations. Insome embodiments, locations in pilot pattern 1271 corresponding to thelocation of the third diagonal line 1281 in pilot pattern 1270 are usedto transmit data symbols. In some embodiments, locations in pilotpattern 1271 corresponding to the location of the third diagonal line1281 in pilot pattern 1270 are partially filled with pilot symbols insuch a manner that the pilot symbol density of pilot pattern 1270 isstill higher than pilot pattern 1270.

More generally, in some embodiments a pilot pattern used fortransmission of MIMO OFDM symbols from an antenna of a group ofcollocated antennas has a higher density of pilot symbols than pilotpatterns being transmitted from the other collocated antennas of thegroup.

In some embodiments the additional line or lines of pilot symbols in thehigher density pilot pattern includes only pilot symbols. In someembodiments the additional line or lines of pilot symbols include nullsymbol locations alternated with the pilot symbols. The null symbollocations may be populated with data symbols when the data symbols areloaded into the pilot patterns. In some embodiments the pilot symbolpattern is repeated for each super slot. In some embodiments the pilotsymbol pattern may be different from one super slot to the next.

In some implementations, base stations in different communication cellseach have a unique diagonal slope for pilot symbols in the pilotpatterns used by multiple antennas. In situations where a mobile stationreceives transmissions from each of two respective base stations ofadjacent cells, if the diagonal lines of the pilot symbols in the pilotpatterns are non-parallel, some pilot symbols transmitted by multiplebase stations may occupy the same frequency and time symbol location.For example, in FIG. 13D, a first pair of pilot patterns 1150, 1151 fora first base station is shown having respective pilot patterns that eachinclude two negative slope diagonal lines, generally indicated at 1155for pilot pattern 1150 and 1157 for pilot pattern 1151, each with arise-over-run equal to −1. A second pair of pilot patterns 1160,1161 fora second base station is shown having respective pilot patterns thateach include three negative slope diagonal lines, generally indicated at1165 for pilot pattern 1160 and 1167 for pilot pattern 1161, each with arise-over-run equal to −2. A receiver receiving both pairs of pilotpatterns 1150,1151 and 1160,1161 would receive pilots at the same OFDMsymbol on the same subcarrier at two symbol locations identified byreference characters 1153 and 1154.

To overcome a problem that would be caused by two pilots occupying thesame location in time and frequency, in some embodiments pre-processingtechniques are used to encode pilot symbols that are varied fordifferent base stations. Each respective base station indicates toreceivers that a particular type of pre-processing is used to encodepilots. In this way a receiver can differentiate received pilotssymbols, even when received at the same OFDM symbol on a samesubcarrier. An example of such pre-processing may involve modifying thepilot symbols with a phase variance that they would not otherwise haveand that is different from the phase variance of the pilot symbolstransmitted by other base stations in other communication cells.

In some embodiments, pre-processing is performed by the pilot inserter23 of FIG. 2 and involves a particular space-time coding pattern for thepilots that is different than a space-time coding pattern for pilotsymbols transmitted by other base stations in other communication cells.

Inserting Data and Pilot Symbols for UL MIMO

FIG. 14A illustrates an example of time-frequency patterns 1600,1601showing data mapping and pilot mapping for UL transmission antennasusing a MIMO OFDM scheme. The time-frequency patterns 1600,1601 are twodimensional plots in which one dimension is a time direction and theother dimension is a frequency direction. In the frequency direction,each discrete horizontal row represents a single sub-carrier. Eachdiscrete vertical column represents an OFDM symbol.

Time-frequency pattern 1600 shows a multiple symbol grouping 1610A usedto transmit symbols on a pair of adjacent subcarriers 1630 in a firstsegment 1640 of seven OFDM symbols before hopping to a different pair ofsubcarriers 1632 for transmission of symbol grouping 1610B in a secondsegment 1650 of seven OFDM symbols. Time-frequency pattern 1600 is fortransmission on a first antenna.

Time-frequency pattern 1601 shows a symbol grouping 1620A used totransmit symbols on an adjacent pair of subcarriers 1634 in the firstsegment 1640 of seven OFDM symbols before hopping to a different pair ofsubcarriers 1636 for transmission of symbol grouping 1620B in the secondsegment 1650 of seven OFDM symbols. Time-frequency pattern 1601 is fortransmission on a second antenna.

In some embodiments, the first and second antennas on whichtime-frequency patterns 1600,1601 are transmitted are respectiveantennas on a single mobile station. In some embodiments, the first andsecond antennas on which time-frequency patterns 1600,1601 aretransmitted are an antenna on a first mobile station and an antenna onsecond mobile station operating in a synchronized, cooperative manner.Using antennas in separate mobiles in a cooperative manner is describedin further detail below.

Symbol grouping 1620A is described above as including symbols on anadjacent pair of subcarriers 1634. In some embodiments, adjacentsubcarriers in the frequency direction are a set of consecutivesubcarriers of an allocated frequency band. In some embodiments,adjacent subcarriers in the frequency direction are a set of subcarriersthat are not necessarily consecutive, but are a selection of subcarriersgrouped together to form a “logical” subband of subcarriers.

Each symbol grouping 1610A,1610B,1620A,1620B in the respective segmentsutilizes a pair of subcarriers for a duration of seven OFDM symbols,then the transmission for the particular mobile station hops to adifferent subcarrier. More generally, the number of OFDM symbols in asegment is implementation specific and is not limited to the particularexample of seven OFDM symbols as illustrated in FIG. 14A.

In some embodiments a hopping sequence is periodic having a perioddefined by a “super slot”, formed by multiple concatenated segments. Thesuper slot of FIG. 14A is shown to have a duration of two segments1640,1650. More generally, it is to be understood that the duration ofthe super slot is implementation specific and can be greater than thetwo segments shown in FIG. 14A. Similarly, the number of subcarriers inthe time-frequency patterns 1600,1601 is implementation specific and canbe greater than or less than the nine subcarriers shown in FIG. 14A.

Symbol groupings 1610A and 1620A are inserted at the same position inthe two respective time-frequency patterns 1600,1601. Similarly, symbolgroupings 1610B and 1620B are inserted at the same position in the tworespective time-frequency patterns 1600,1601.

During insertion of the symbols, a transmission data stream 1602, whichfor example includes symbols “. . . S₂₄, . . . S₂, S₁”, is divided intofirst data stream 1604 including odd-numbered symbols “. . . S₂₃, . . .S₃, S₁” and second data stream 1606 including even-numbered symbols “. .. S₂₄, . . . S₄, S₂”. The first and second data streams 1604,1606 aremapped to the time-frequency patterns 1600,1601 respectively. Theodd-numbered symbols “. . . S₂₃ . . . S₃, S₁” are mapped totime-frequency pattern 1600 for transmission on sub-carriers of thefirst antenna and the even-numbered symbols “. . . S₂₄, . . . S₄, S₂”are mapped to time-frequency pattern 1601 for transmission onsub-carriers of the second antenna. One of ordinary skill in the artwill recognize that the transmission data stream 1602 can be mapped tomore than two antennas by separating the transmission date stream intomore than two streams and mapping to more than two time-frequencypatterns.

For each time-frequency pattern, the respective data streams 1604 and1606 are allocated using symbol groupings, for example 1610A and 1610Bwhich each occupy seven OFDM symbols. Odd-numbered symbols “S₂₃, S₂₁,S₁₉, S₁₇, S₁₅, S₁₃, S₁₁, S₉, S₇, S₅, S₃, S₁” are mapped totime-frequency pattern 1600 as one symbol grouping 1610B. Even-numberedsymbols “S₂₄, S₂₂, S₁₈, S₁₆, S₁₄, S₁₃, S₁₂, S₁₀, S₈, S₆, S₄, S₂” aremapped to time-frequency pattern 1601 as one symbol grouping 1620B. Oneof ordinary skill in the art would recognize that other arrangements forallocating the symbols are possible. Furthermore, the present inventionis not limited to the use of two antennas for transmission of datapackets on the up link transmission channel. Illustration oftime-frequency patterns for use with two antennas is employed simply asan example, and is not intended to limit the scope of the invention.

FIG. 14A shows only a portion of the time-frequency patterns 1600,1601filled with symbols for transmission. It is to be understood that datasymbols can fill some or all of the available symbol locations in thetime-frequency patterns 1600,1601 after pilot symbols are located in thetime-frequency pattern. FIG. 14A shows only locations filled in thetime-frequency patterns to be transmitted by a respective pair ofantennas from a single mobile. Other mobiles are transmittingtime-frequency patterns having a similar time-frequency structure, butin a manner that provides that data and pilots of the single mobile andthe other mobiles do not interfere with one another. Furthermore, whilesymbols for transmission on a single data stream 1602 are shown in FIG.14A to be divided into two data streams 1604,1606, it is to beunderstood that more than two time-frequency patterns can be generatedfrom each of one or more data streams.

Only a single pair of subcarriers 1630,1634 is utilized in each segmentfor each data stream 1604,1606. However, it is to be understood that adata stream could be mapped to more one than one pair of subcarriers ineach segment.

A hopping sequence is formed by multiple concatenated segments, eachsegment including symbol groupings hopping amongst differentsubcarriers. While FIG. 14A shows only two concatenated segments1640,1650 with a single frequency hop occurring for each data stream1604,1606, it is to be understood that the number of segments in ahopping sequence is implementation specific and may be greater than orless than the two segments 1550,1560 shown in FIG. 14A.

In some embodiments the symbol groupings are not on immediately adjacentsubcarriers, but the subcarriers may be spaced apart by a particularnumber of subcarriers. In some embodiments the spacing of the pairs ofsubcarriers, either immediately adjacent or spaced apart, are maintainedfor all concatenated segments forming a periodic super slot. In someembodiments the spacing of the pairs of subcarriers, either immediatelyadjacent or spaced apart, varies from segment to segment in each of theconcatenated segments forming a periodic super slot.

In some embodiments the hopping sequences are different in adjacentcommunication cells.

The symbol groupings 1610A,1610B,1620A,1620B for each mobile station ofthe time-frequency pattern 1600,1601 include data symbols as well aspilot symbols.

In the example of time-frequency pattern 1600, in the first segment 1640a pilot symbol 1615 is inserted on one of the subcarriers of the pair ofsubcarriers 1630 and a null symbol location 1617 is inserted on theother subcarrier of the pair of subcarriers. Both the pilot symbol 1615and null symbol location 1617 are inserted in the same OFDM symbol, thatis the fourth OFDM symbol of the seven OFDM symbols forming the durationof the first segment 1640. A similar pattern is found in the secondsegment 1650 for a pilot symbol and null symbol location in symbolgrouping 1610B.

In time-frequency pattern 1601, a similar pattern is found for a pilotsymbol 1616 and null symbol location 1618, except that the pilot symbol1616 and null symbol location 1618 are on opposite subcarriers of thepair of subcarriers 1634 than the pilot symbol 1615 and null symbollocation 1617 of time-frequency pattern 1600.

In the illustrated example, the pilot symbol and null symbol locationare shown in both segments 1640,1650 in both time-frequency patterns1600,1601 to be inserted at the fourth OFDM symbol. It is to beunderstood that the location of the pilot symbol and null symbollocation is implementation specific and they may occur on any of theOFDM symbols in a given segment. In some embodiments the pilot symboland null symbol location are inserted in a same OFDM symbol position forthe same data stream in each respective segment, for example frequencyhopping symbol groupings 1610A,1610B. In some embodiments the pilotsymbol and null symbol location are inserted in a different OFDM symbolposition for the same data stream in each respective segment.

In the illustrated example, the pilot symbol 1615 is shown to occur onthe first subcarrier of the pair of subcarriers and the null symbollocation 1617 to occur on the second subcarrier of the pair ofsubcarriers of symbol groupings 1610A,1610B in time-frequency pattern1600 and the null symbol location 1618 is shown to occur on the firstsubcarrier of the pair of subcarriers and the pilot symbol 1616 to occuron the second subcarrier of the pair of subcarriers of symbol grouping1620A,1620B in time-frequency pattern 1601. In some embodiments theinsertion of the pilot symbol and the null symbol location changepositions in the pair of subcarrier for some or all segments. Forexample, FIG. 14B shows a single segment for each of a pair oftime-frequency patterns 1660,1661 similar to segment 1640 of FIG. 14A.The segment is seven OFDM symbols by nine subcarriers. Time-frequencypattern 1660 is for example for transmission on a first antenna andtime-frequency pattern 1661 is for example for transmission on a secondantenna. In each of the pair of time-frequency patterns 1660,1661, apair of subcarriers 1665,1666, namely the fifth and sixth subcarriersare used for transmitting data and pilot symbols in a similar manner toFIG. 14A. In the first time-frequency pattern 1660 a pilot symbol 1670is located in the third OFDM symbol location of the first subcarrier ofthe pair of subcarriers 1665 and a null symbol location 1671 is locatedin the fourth OFDM symbol location of the second subcarrier of the pairof subcarriers 1665. In the second time-frequency pattern 1661 a pilotsymbol 1675 is located in the fourth OFDM symbol location of the firstsubcarrier of the pair of subcarriers 1666 and a null symbol location1676 is located in the third OFDM symbol location of the secondsubcarrier of the pair of subcarriers 1666.

Referring to FIG. 14A, each symbol grouping 1610A,1610B,1620A,1620B inthe respective segments 1640,1650 utilizes a pair of subcarriers for aduration of seven OFDM symbols, then the transmission for a data streamhops to a different subcarrier. More generally, the number of OFDMsymbols in a segment is implementation specific and is not limited tothe particular example of seven OFDM symbols as illustrated in FIG. 14A.

In some embodiments a hopping sequence is periodic having a perioddefined by a “super slot” that includes two or more segments. The superslot of FIG. 14A is shown to have a duration of two segments 1640,1650.More generally, it is to be understood that the duration of the superslot is implementation specific and can be greater than the two segmentsshown in FIG. 14A. Similarly, the number of subcarriers in thetime-frequency patterns 1600,1601 is implementation specific and can begreater than or less than the nine subcarriers shown in FIG. 14A.

In some embodiments, at least one pilot symbol is mapped in the symbolgrouping 1610A,1610B for each segment 1640. In some embodiments, pilotsymbols are not included in each segment. In such situations,interpolation can be performed to estimate channel characteristics forthose segments not having a pilot symbol by using the pilot symbols fromadjacent segments.

In some embodiments of the invention, a pair of UL pilots, one pilot ineach of the time-frequency patterns 1600,1601, corresponds to a pilotfor each transmission antenna. This may occur when a data stream from arespective mobile station is mapped to multiple antennas of that mobilestation. In some embodiments of the invention, a pair of UL pilots, onepilot in each of the time-frequency patterns 1600,1601, corresponds to apilot for each respective mobile station. This may occur for two or moreantenna operating in combined manner when a data stream from a firstmobile station is mapped to only a single antenna of the first mobilestation and a data stream from a second mobile station is mapped to onlya single antenna of the second mobile station. Those skilled in the artwill also be aware that the data streams from any number of users can bemapped to any number of transmission antennas.

In some embodiments, a frequency hopping pattern for pilot symbols, inwhich a transmission frequency for a given data stream hops after amultiple of OFDM symbols, is created by inserting one or more pilotsymbols in a plurality of OFDM symbols on a different pair ofsubcarriers of an allocated transmission bandwidth than a pair ofsubcarriers used prior to the most recent frequency hop.

FIG. 14A illustrates inserting data symbols in time-frequency patterns1600,1601 in a similar manner to that of FIG. 12A. A data stream ofsymbols “S₁, S₂, . . . S₂₄” 1602 is divided into respective streams ofodd-numbered and even-numbered symbols 1604,1606. The odd-numberedsymbols 1604 are mapped to the pair of sub-carriers 1632 intime-frequency pattern 1600 and the even-numbered symbols 1606 aremapped to the pair of sub-carriers 1636 in time-frequency pattern 1601.

FIGS. 12 and 14A illustrate a single data stream being divided intofirst and second data streams that are each mapped to respectivetime-frequency patterns. However, in some implementations, data streamsfor two different transmitters are mapped to respective time-frequencypatterns for transmission on antennas of the two different transmitters.This form of combined MIMO transmission of a synchronized time-frequencypattern from antennas on at least two different transmitters is referredto as virtual MIMO. For example, two mobile stations, each having asingle antenna are coordinated to each provide a time-frequency patternin a similar fashion that a single transmitter provides twotime-frequency patterns for each of two antennas from one or more datastreams. Similarly, two base stations in adjacent cells could eachdedicate a single antenna to communicate with at least one mobilestation that is close to the cell boundary using a virtual MIMO scheme.

In some embodiments for DL and/or UL transmission, the pilot pattern iscyclically offset, in a time direction and/or in a frequency direction,to form re-use patterns. For example, multiple time-frequency patternsemploy the same arrangement of pilot symbols, but respectivearrangements are offset in at least one of time and frequency for one ormore of the multiple time-frequency patterns for use by different mobilestations. In some embodiments, time-frequency patterns can be selectedfrom the multiple time-frequency patterns for use by different mobilestations to avoid interference between mobile stations transmitting tothe same base station. In some embodiments, time-frequency patterns canbe selected from the multiple time-frequency patterns for use bydifferent base stations to avoid interference between base stationstransmitting in adjacent cells. Therefore, in some embodiments, the basestation and/or mobile stations of adjacent communication cells use pilotpatterns that are the same pattern, but are cyclically offset in timeand/or frequency with respect to one another.

In some embodiments for DL and/or UL transmission, the pilot symbols aretransmitted with a power level greater than a power level of datasymbols, depending upon a value reflective of channel conditions.

With reference to FIG. 15, a general method for implementing insertionof data symbols and pilot symbols into an OFDM transmission resource tocreate the above-described time-frequency patterns will now bedescribed. The method can be used for creating DL time-frequencypatterns and/or UL time-frequency patterns. A first step 1710 involvesfor each antenna, inserting pilot symbols in a respective pattern intime-frequency in which the pilot symbols for each antenna are insertedsuch that that pilot symbols from other antennas do not occupy the samelocation in time-frequency. A second step 1720 involves, also for eachantenna, inserting data symbols are in an identical frequency-hoppingpattern in time-frequency.

In some embodiments, the method is applied to a transmitter in which thenumber of antennas is two. In some embodiments, the method is applied toa transmitter in which the number of antennas is greater than two.

In some embodiments, the method is used for inserting data symbols andpilot symbols for DL signaling between a base station and one or moremobile stations. Examples of such methods will be described with regardto FIGS. 16A, 16B and 16C.

In some embodiments pilot symbols are inserted in a DL time-frequencypattern such that each pilot symbol is offset from a previous pilotsymbol in at least one of a time and a frequency direction in a samedirection as the previous pilot symbol is from all previously insertedpilot symbols so that the pilot symbols form at least one diagonal linein the time-frequency pattern.

A manner in which pilot symbols are inserted in time-frequency so thatpilot symbols do not occupy the same location in time-frequency ondifferent antennas for when the number of antennas is equal to two willbe described with respect to FIG. 16A. At step 1710A for each of theantennas, pilots are inserted in the DL time-frequency pattern byalternating insertion of null symbol locations and pilot symbols in atleast one diagonal line for a first antenna of the pair of antennas andalternating insertion of pilot symbols and null symbol locations in atleast one diagonal line for a second antenna of the pair of antennas.The null symbol locations of the first antenna correspond to a samelocation in time-frequency as the pilot symbols of the second antenna,and vice versa. Step 1720 is the same as step 1720 of FIG. 15.

FIG. 16B illustrates another embodiment of inserting pilot symbols. Atstep 1710B, for each antenna of a pair of antennas, pilot symbols areinserted in a first diagonal line in time-frequency and null symbollocations are inserted in a second diagonal line in time-frequency. Thefirst diagonal line and the second diagonal line are offset by aconstant distance in time-frequency and the null symbol locations of afirst antenna of the pair of antennas occur at a same location intime-frequency as the pilot symbols of a second antenna of the pair ofantennas, and vice versa. Step 1720 is the same as step 1720 of FIG. 15.

With regard to step 1710 of FIG. 15, step 1710A of FIG. 16A and 1710B ofFIG. 16B, in some embodiments, a larger number of pilot symbols areinserted in the time-frequency pattern of at least one antenna of agroup of antennas such that the density of pilot symbols for the atleast one antenna is higher than for other antennas in the group.

Referring to FIG. 16C, a method will now be described for inserting datasymbols in an identical frequency hopping pattern. Step 1710 is the sameas step 1710 of FIG. 15. At step 1720A, for each antenna when the numberof antennas is equal to two, data symbols are inserted on a set ofspaced apart subcarriers that change each symbol duration of a pluralityof symbol durations. In other words, for each frequency hop, at leastone data symbol of a series of data symbols is inserted on a set ofdifferent subcarriers of an allocated transmission bandwidth than a setof subcarriers used prior to a most recent frequency hop for a previousat least one data symbol of the series of data symbols. In someembodiments, frequency-hopping occurs for data symbols in a group ofOFDM durations on one or more sub-carriers, as opposed to occurring foreach OFDM symbol.

FIGS. 16A, 16B and 16C are described above as being methods for use withtwo antennas. More generally, the methods can be used for more than twoantennas.

In some embodiments, the method is used for inserting data symbols andpilot symbols for UL signaling between one or more mobile stations and abase station. Examples of such methods will be described with regard toFIGS. 17A and 17B.

A manner in which pilot symbols are inserted in time-frequency so thatpilot symbols do not occupy the same location in time-frequency ondifferent antennas for when the number of antennas is equal to two willbe described with respect to FIG. 17A. At step 1710C, for each antennain a pair of antennas, a null symbol location and a pilot symbol areinserted for a first antenna of the pair of antennas and a pilot symboland a null symbol location are inserted for a second antenna of the pairof antennas, such that the null symbol location of the first antenna isinserted at the same location in time-frequency as the pilot symbol ofthe second antenna, and vice versa. Step 1720 is the same as step 1720of FIG. 15.

Referring to FIG. 17B, a method will now be described for inserting datasymbols in an identical frequency hopping pattern. Step 1710 is the sameas step 1710 of FIG. 15. At step 1720B, for each antenna when the numberof antennas is equal to two, data symbols are inserted on a set ofsubcarriers that is constant over a set of consecutive symbol durations,and change for each set of multiple sets of consecutive symbol duration.That is for each frequency hop, a plurality of data symbols of theseries of data symbols is inserted in a corresponding plurality of OFDMsymbols on one or more subcarriers of an allocated transmissionbandwidth than one or more subcarriers used prior to a most recentfrequency hop for a previous plurality of data symbols of the series ofdata symbols.

FIGS. 17A and 17B are described above as being methods for use with twoantennas. More generally, the methods can be used for more than twoantennas.

A transmitter used to implement some embodiments of the invention mayinclude a plurality of transmit antennas, an encoder for inserting datasymbols in an identical frequency-hopping pattern in time-frequency foreach of the plurality of antennas. In some embodiments the transmitterincludes a pilot inserter for inserting pilot symbols in a respectivepattern in time-frequency for each of the antennas, wherein the pilotsymbols for each antenna are inserted such that pilot symbols from otherantennas do not occupy the same location in time-frequency. In someembodiments the transmitter is of the form of the transmitter shown inFIG. 2, in which the encoder is encoder 14 and the pilot inserter ispilot inserter 23.

A receiver used to implement some embodiments of the invention receivesOFDM symbols including pilot symbols in a respective pattern intime-frequency and data symbols in an identical frequency-hoppingpattern in time-frequency, the pilot symbols for each respective patternin time-frequency inserted such that pilot symbols from differentantennas do not occupy the same location in time-frequency and forreceiving information identifying a particular form of pre-processingused to encode the received pilot symbols from at least one source. Insome embodiments the receiver is of the form of the receiver shown inFIG. 3.

In some embodiments, the receiver further includes logic fordifferentiating pilots from different sources. In some embodiments,differentiating pilot logic is included as a part of channel estimationfunctionality, for example channel estimator 72 of FIG. 3. As describedabove received symbols are passed to channel estimator 72, whichanalyses received pilots symbols located at known times and frequencieswithin the OFDM frame. In some embodiments, the differentiating pilotlogic is a separate functionality from the channel estimationfunctionality. In some embodiments, the differentiating pilot logicutilizes the information identifying the particular form ofpre-processing used to encode the received pilot symbols todifferentiate between received pilot symbols from different sourcesoccurring at a same time-frequency location.

What has been described is merely illustrative of the application of theprinciples of the invention. Other arrangements and methods can beimplemented by those skilled in the art without departing from thespirit and scope of the present invention.

1. An OFDM receiver, comprising: at least one receive antenna operableto receive OFDM symbols transmitted from a plurality of transmitantennas, the OFDM symbols transmitted from each transmit antennacomprising pilot symbols in a respective pattern in time-frequency anddata symbols in an identical frequency-hopping pattern intime-frequency, the pilot symbols for each respective pattern intime-frequency inserted such that pilot symbols from different transmitantennas do not occupy the same location in time-frequency; and achannel estimator operable to compare the received pilot symbols topilot symbol values known to be transmitted by a transmitter to estimatea channel between the transmitter and the receiver.
 2. The OFDM receiveras defined in claim 1, wherein the channel estimator is operable foreach transmit antenna/receive antenna combination: to estimate a channelresponse for each point in a respective pattern using the received pilotsymbols; to estimate a channel response for each of a plurality ofpoints not in the respective pattern by performing a two-dimensional(time direction, frequency direction) interpolation of channel responsesdetermined for points in the respective pattern; and to perform aninterpolation in the frequency direction to estimate channel responsescorresponding to remaining OFDM sub-carriers within each OFDM symbol. 3.The OFDM receiver as defined in claim 2, wherein the channel estimatoris operable to filter the channel responses prior to interpolating inthe frequency direction to estimate the channel responses correspondingto remaining OFDM sub-carriers within each OFDM symbol.
 4. The OFDMreceiver as defined in claim 3, wherein the channel estimator isoperable to filter the channel responses by performing a three-pointsmoothing operation.
 5. The OFDM receiver as defined in claim 2 whereinthe channel estimator is operable to estimate the channel response of aplurality of points not in the respective pattern by performing atwo-dimensional interpolation of channel responses for each of aplurality of points in the respective pattern by, for each sub-carrierto be estimated, averaging: a channel response of the given channelestimation period of a sub-carrier before the sub-carrier to beestimated in frequency (when present); a channel response of the givenchannel estimation period of a sub-carrier after the sub-carrier to beestimated in frequency (when present); a channel response for a previousestimation period (when present); and a channel response for a followingestimation period (when present).
 6. The OFDM receiver as defined inclaim 2, wherein the channel estimator is operable to perform aninterpolation in the frequency direction by: performing a linearinterpolation for sub-carriers at a lowest useful frequency or a highestuseful frequency within the OFDM symbol; and performing a cubic Lagrangeinterpolation for sub-carriers not at the lowest or highest usefulfrequency or the highest useful frequency.
 7. The OFDM receiver asdefined in claim 1, wherein the OFDM symbols transmitted from eachantenna comprise information identifying a particular form ofpre-processing used to encode the received pilot symbols from at leastone source, the OFDM receiver further comprising differentiating pilotlogic operable to process the information identifying the particularform of pre-processing used to encode the received pilot symbols todifferentiate between received pilot symbols from different sources. 8.The OFDM receiver as defined in claim 7, wherein the differentiatingpilot logic is operable to process the information identifying theparticular form of pre-processing used to encode the received pilotsymbols to differentiate between received pilot symbols from differentsources occurring at a same time-frequency location.
 9. A method ofprocessing OFDM symbols transmitted from a plurality of transmitantennas, the OFDM symbols transmitted from each transmit antennacomprising pilot symbols in a respective pattern in time-frequency anddata symbols in an identical frequency-hopping pattern intime-frequency, the pilot symbols for each respective pattern intime-frequency inserted such that pilot symbols from different transmitantennas do not occupy the same location in time-frequency, the methodcomprising: receiving the OFDM symbols on at least one receive antenna;and comparing the received pilot symbols to pilot symbol values known tobe transmitted by a transmitter to estimate a channel between thetransmitter and the receiver.
 10. The method as defined in claim 9,further comprising, for each transmit antenna/receive antennacombination: estimating a channel response for each point in arespective pattern using the received pilot symbols; estimating achannel response for each of a plurality of points not in the respectivepattern by performing a two-dimensional (time direction, frequencydirection) interpolation of channel responses determined for points inthe respective pattern; and performing an interpolation in the frequencydirection to estimate channel responses corresponding to remaining OFDMsub-carriers within each OFDM symbol.
 11. The method as defined in claim10, further comprising filtering the channel responses prior tointerpolating in the frequency direction to estimate the channelresponses corresponding to remaining OFDM sub-carriers within each OFDMsymbol.
 12. The method as defined in claim 11, wherein the step offiltering the channel responses comprises performing a three-pointsmoothing operation.
 13. The method as defined in claim 10, wherein thestep of estimating the channel response of a plurality of points not inthe respective pattern by performing a two-dimensional interpolation ofchannel responses for each of a plurality of points in the respectivepattern comprises, for each sub-carrier to be estimated, averaging: achannel response of the given channel estimation period of a sub-carrierbefore the sub-carrier to be estimated in frequency (when present); achannel response of the given channel estimation period of a sub-carrierafter the sub-carrier to be estimated in frequency (when present); achannel response for a previous estimation period (when present); and achannel response for a following estimation period (when present). 14.The method as defined in claim 10, wherein the step of performing aninterpolation in the frequency direction comprises: performing a linearinterpolation for sub-carriers at a lowest useful frequency or a highestuseful frequency within the OFDM symbol; and performing a cubic Lagrangeinterpolation for sub-carriers not at the lowest or highest usefulfrequency or the highest useful frequency.
 15. The method as defined inclaim 9, wherein the OFDM symbols further comprise informationidentifying a particular form of pre-processing used to encode thereceived pilot symbols from at least one source, the method furthercomprising processing the information identifying the particular form ofpre-processing used to encode the received pilot symbols todifferentiate between received pilot symbols from different sources. 16.The method as defined in claim 16, wherein the step of processing theinformation identifying the particular form of pre-processing used toencode the received pilot symbols to differentiate between receivedpilot symbols from different sources differentiates between receivedpilot symbols from different sources occurring at a same time-frequencylocation.